Gas trapped in the pump housing of a centrifugal pump impairs the efficiency of the pump, and may cause the pump to not work at all. In some circumstances, the pump may fail prematurely. To deal with these problems, there have been many attempts to remove gas from the pump housing of a centrifugal pump, either prior to startup, or during the startup phase. One straightforward solution has been to situate the pump in such a manner that the pump housing is filled with liquid after the outlet is covered, and before the pump is started. In some instances, however, this orientation of the pump may not be possible due to space constraints. Thus, the outlet of a pump may be lower than the top of the pump impeller; in this situation air may be trapped above the inlet pipe and in the suction and discharge chambers.
It is also recognized that gas may sometimes enter the pump chamber, as for example, if the suction head is lost between pumping cycles. In general, only after continuous operation of a pump will air or gas typically be purged from the pump housing, with resultant efficient pump operation. It is important, therefore, that air be purged from centrifugal pumps and other pumps that are susceptible to losing prime.
Before pump startup, both liquid and air are typically contained in the suction and discharge chambers of a centrifugal pump. When the pump motor is energized to start rotation of the impeller, a quantity of the liquid and gas from the suction chamber may be moved into the pump discharge chamber. This action creates a partial vacuum on the suction side of the pump. Atmospheric pressure exerted on the liquid to be pumped out of the suction chamber, coupled with the internal dimension of the inlet pipe, causes the fluid to rise into the inlet pipe. If the rotation of the impeller continues to move quantities of liquid and gas from the suction chamber into the discharge chamber, the height of the liquid in the pump chamber progressively increases. The priming cycle is completed when a vacuum has been established in the suction chamber, and the pump has been primed and is operating at its rated capacity and head, because most of the gas has been removed from the suction chamber. As noted above, however, there are instances when a pump may not prime and may eventually fail. Such is frequently the case, for example, in the operation of hydrotherapy tubs.
Hydrotherapy tubs typically have one or more jets which jet water against portions of a person who is occupying the tub. Water in the tub is circulated by means of a centrifugal pump from an exit near the bottom of the tub to a discharge line which leads to exit ports in one or more jets located in the tub. Water generally fills the tub to a point above the pump. The jet exit ports are generally located below the discharge or outlet connection of the pump. It is this vertical relationship between the exit ports and the outlet connection of the pump which causes air to be trapped in the pump and makes the pump difficult to prime.
In a hydrotherapy tub, the priming cycle of its pump begins while the tub reservoir is being filled. Water passes into the pump's piping system via the suction line of the pump, and rises above the elevation of the exit ports of the hydrotherapy jets. When the exit ports are covered with water, however, gas within the fluid piping system is unable to escape. As noted above, if the centrifugal pump is located above the level of the exit ports, or even if it is level with these ports, air will be trapped inside of the centrifugal pump making it difficult to prime.
In many cases of hydrotherapy tubs and in other instances as well, it may be impossible to locate the centrifugal pump below the surface of the exit ports. Thus, as will be apparent from the foregoing description it is necessary in this case to vent the air or other gas that is trapped in the pump housing in order to prime the pump. Otherwise, the air or gas prevents water from completely filling the pump housing, and the pump impeller cannot efficiently pump water into the discharge chamber even after the priming cycle. Pressure in the discharge chamber, therefore, remains relatively low. Unless the gas is completely vented, the pump is not able to develop sufficient head or operate at rated capacity.